2. Determine the heat of reaction (AH, xn) for the process by which hydrazine (N2H4) is formed from its elements:
N2 (g) +
2 H2 (g) →
N2H4 (g)
by using the following thermochemical data:
N H. (g)
+
O2 (g) →
N2 (g)
-
2 H2O(g)
AH - - 622.2 kJ
H2(g) +
1/2 O2 (g)
H2O (g)
AH = - 285.8 kJ
IN

The heat of reaction : 50.6 kJ

Based on the principle of Hess's Law, the change in enthalpy of a reaction will be the same even though it is through several stages or ways

Reaction

N₂(g) + 2H₂(g) ⇒N₂H₄(l)

thermochemical data:

1. N₂H₄(l)+O₂(g)⇒N₂(g)+2H₂O(l)   ΔH=-622.2 kJ

2. H₂(g)+1/2O₂(g)⇒H₂O(l)  ΔH=-285.8 kJ

We arrange the position of the elements / compounds so that they correspond to the main reaction, and the enthalpy sign will also change

1. N₂(g)+H₂O(l) ⇒  N₂H₄(l)+O₂(g) ΔH=+622.2 kJ

2. H₂(g)+1/2O₂(g)⇒H₂O(l)  ΔH=-285.8 kJ x 2 ⇒

2H₂(g)+O₂(g)⇒2H₂O(l)  ΔH=-571.6 kJ

Add reaction 1 and reaction 2, and remove the same compound from different sides

1. N₂(g)+2H₂O(l) ⇒  N₂H₄(l)+O₂(g) ΔH=+622.2 kJ

2.2H₂(g)+O₂(g)⇒2H₂O(l)            ΔH=-571.6 kJ

+

N₂(g) + 2H₂(g) ⇒N₂H₄(l)   ΔH=50.6 kJ

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